I can't speak for this situation, but before I moved to tech I worked in a heavily union industry, and all the union shops had in their bargaining contract that the employer could not hire anyone who wasn't in the union.

For people in the union it was great. For people not in the union, it sucked. And before somebody says, "well just join the union" (do you really think I'm that stupid not to have thought about that?) joining the union was really, really hard. You basically had to wait for somebody to either retire or die, because nobody ever left for other reasons, and because employees were super expensive and impossible to fire, union shops were extremely conservative with hiring. Union employees got paid double what non-union did and had 1/4 of the work to do, plus they slacked off like crazy and never got in trouble for it. The union was (or at least looked to us) like it was great for the employees. I'd hate to be the employer though.